the plot right now is luffy trying to reach the top of the skull to help the samurai but can't because BM on his way
any sensible person would say he cant defeat BM then go to Kaido, here where pero and marco alliance play its role
but Marco to succeeding in making BM go away need to be impactful , BM getting humiliated then just leaving would just seem like Marco achieved nothing and SHs didn't even need his help
so logically BM need to put SHs in a bad situation then Marco save the day